1. Field of the Invention
The present invention relates to a polishing composition useful for polishing the surface of a substrate for a magnetic disk to be used for a memory hard disk, i.e. a memory device useful for e.g. a computer, and a polishing method employing it.
More particularly, the present invention relates to a polishing composition and a polishing method employing it, in polishing of a disk substrate (hereinafter referred to as substrate) to be used for a memory hard disk represented by e.g. a Nixe2x80x94P disk, a Nixe2x80x94Fe disk, an aluminum disk, a glass disk, a boron carbide disk or a carbon disk, whereby polishing can be carried out without increasing edge sagging at the periphery portion (hereinafter referred to as xe2x80x9cedge portionxe2x80x9d) as compared with a substrate polished by a conventional polishing composition, and a magnetic disk device having a higher capacity and a higher memory density can be produced.
2. Discussion of Background
There have been continuing efforts for miniaturization, larger capacity and lower price for memory hard disks to be used for a magnetic disk device, which are one of memory media for e.g. computers. One of substrates which are most widely used at present, is one having an electroless Nixe2x80x94P plating film formed on a blank material. The blank material is one obtained by fairing an aluminum or another base plate as a base of the substrate by processing by diamond turning, lapping by means of a PVA grind stone prepared by fixing SiC grinding material or other methods, for the purpose of parallelization or planarization. However, by such various fairing methods, a relatively large waviness can not completely be removed. And, the electroless Nixe2x80x94P plating film will be formed along the waviness on the blank material. Accordingly, such a waviness will remain also on the substrate in some cases. Accordingly, for the purpose of removing the waviness of the substrate and making the surface smooth and flat, surface polishing is carried out.
Along with the increase in the capacity of memory hard disks, the surface storage density is increasing at a rate of a few tens percent per year. Accordingly, the space on a memory hard disk occupied by a predetermined amount of recorded information, is lower than ever, and the magnetic force required for storage tends to be weak. Accordingly, recently, it is required to minimize a flying height of the head, which is a space between the magnetic head and the memory hard disk, in a magnetic disk device in recent years.
Further, so-called texturing may sometimes be carried out to impart concentric circular scorelines to the substrate after polishing, for the purposes of preventing sticking of the magnetic head for reading or writing information, to the memory hard disk and preventing non-uniformity of the magnetic field on the memory hard disk due to scorelines in a certain direction different from the rotational direction of the memory hard disk, formed on the substrate surface by polishing.
Recently, for the purpose of further reducing the flying height of the head, light texturing is carried out wherein the scorelines formed on the substrate are further reduced, or a non-texture substrate free from scorelines, is employed which is not subjected to texturing. The technology to support such a low flying height of the magnetic head has also been developed, and the reduction of the flying height of the head is being increasingly advanced. Accordingly, the flying height of the head is reduced to a level of not higher than 0.02 xcexcm.
A magnetic head flies along the shape on the surface of a memory hard disk which rotates at an extremely high speed, and if a pit at a level of several xcexcm is present on a memory hard disk surface, it is likely that information can not completely be written in, thus leading to a defect of information so-called a xe2x80x9cbit defectxe2x80x9d or failure in reading the information, which causes an error.
Here, the xe2x80x9cpitsxe2x80x9d are dents which are originally present on the substrate, or dents formed by polishing on the surface of the substrate, and fine pits are dents having a diameter of less than about 50 xcexcm, among them.
Accordingly, it is important to minimize the roughness of the substrate surface in the step prior to forming a magnetic medium, i.e. polishing step, and at the same time, it is necessary to remove microprotrusions, fine pits and other surface defects effectively with a high efficiency.
For such a purpose, it used to be common to carry out polishing by means of a polishing composition comprising aluminum oxide or other various abrasives and water as well as various polishing accelerators. For example, JP-A-61-278587 and JP-A-62-25187 disclose a polishing composition for a memory hard disk, obtained by adding e.g. aluminum nitrate, nickel nitrate or nickel sulfate as a polishing accelerator to water and aluminum hydroxide, followed by mixing to obtain a slurry.
Further, JP-A-2-84485 discloses an acidic polishing composition for an aluminum magnetic disk which comprises water and an alumina abrasive powder as well as gluconic acid or lactic acid as a polishing accelerator and colloidal alumina as a surface modifier, JP-A-7-133477 discloses an aqueous polishing composition comprising an alumina abrasive, colloidal alumina and an alkali nitrite, and JP-A-9-316430 discloses a polishing composition for a magnetic disk substrate which comprises water, xcex1-alumina and a polishing accelerator, wherein the polishing accelerator is aluminum oxalate.
However, in the case where a substrate is polished by means of the above-mentioned conventional polishing composition, the edge portion of the substrate is more significantly polished than the flat surface during the polishing, thus causing edge sagging at the periphery portion on the substrate. On a substrate having sagging at the edge portion, it is impossible to store data on said edge portion, whereby data storage region decreases.
Accordingly, in production of a magnetic disk device having a high capacity and a high storage density, the edge sagging at the edge portion is problematic, and it is desired to increase storage capacity by reducing the edge sagging so that the edge portion can effectively be used to further increase the storage area.
It is found that the edge sagging is significantly influenced by the polishing composition as well as polishing conditions during the polishing. Accordingly, a polishing composition has been desired with which a magnetic disk device having a high capacity and a high storage density can be obtained without increasing the edge sagging at the edge portion during the polishing.
Here, for evaluation of the edge sagging, the shape of the edge portion at which edge sagging is formed and the depth (sagging width) are obtained, and basically two measured values are employed. Namely, the edge sagging is evaluated by two measured values Roll-Off and Dub-Off as mentioned hereinafter in many cases. However, although the object to be measured is the same with respect to the two measured values, the method for measuring the shape and the standard value as the tolerance limit in the measuring method based on the measured value are not generally determined substantially, depending on difference in characteristics of measuring apparatus to be used, and the standard value is dependent on judgment by an individual manufacturer in many cases.
It is an object of the present invention to solve the above-mentioned problems, and to provide a polishing composition and a polishing method employing it, with which a magnetic disk device having a high capacity and a high storage density can be obtained without increasing edge sagging as compared with a conventional polishing composition, in polishing of a substrate to be used for a memory hard disk.
The present invention provides a polishing composition which comprises the following components:
(a) water,
(b) at least one compound selected from the group consisting of a polyoxyethylene polyoxypropylene alkyl ether and a polyoxyethylene polyoxypropylene copolymer,
(c) at least one compound selected from the group consisting of nitric acid, nitrous acid, sulfuric acid, hydrochloric acid, molybdic acid, sulfamic acid, glycine, glyceric acid, mandelic acid, malonic acid, ascorbic acid, glutamic acid, glyoxylic acid, malic acid, glycolic acid, lactic acid, gluconic acid, succinic acid, tartaric acid, maleic acid and citric acid, and their salts, and
(d) at least one abrasive selected from the group consisting of aluminum oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium oxide, silicon nitride and silicon carbide.
The present invention further provides the polishing composition, wherein the component (c) is at least one member selected from the group consisting of aluminum nitrate, nickel nitrate, lithium nitrate, sodium nitrate, potassium nitrate, iron (III) nitrate, sodium nitrite, potassium nitrite, aluminum sulfate, nickel sulfate, lithium sulfate, sodium sulfate, iron (III) sulfate, ammonium sulfate, aluminum chloride, iron (III) chloride, ammonium chloride, sodium molybdate, ammonium molybdate, nickel sulfamate and ammonium sulfamate.
The present invention further provides the polishing composition, wherein the component (c) is at least one member selected from the group consisting of glycine, glyceric acid, mandelic acid, malonic acid, ascorbic acid, glutamic acid, glyoxylic acid, malic acid, glycolic acid, lactic acid, gluconic acid, succinic acid, tartaric acid, maleic acid and citric acid.
The present invention further provides a polishing method by means of a polishing composition comprising the following components:
(a) water,
(b) at least one compound selected from the group consisting of a polyoxyethylene polyoxypropylene alkyl ether and a polyoxyethylene polyoxypropylene copolymer,
(c) at least one compound selected from the group consisting of nitric acid, nitrous acid, sulfuric acid, hydrochloric acid, molybdic acid, sulfamic acid, glycine, glyceric acid, mandelic acid, malonic acid, ascorbic acid, glutamic acid, glyoxylic acid, malic acid, glycolic acid, lactic acid, gluconic acid, succinic acid, tartaric acid, maleic acid and citric acid, and their salts, and
(d) at least one abrasive selected from the group consisting of aluminum oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium oxide, silicon nitride and silicon carbide.
The present invention further provides the polishing method by means of a polishing composition, wherein the component (c) is at least one member selected from the group consisting of aluminum nitrate, nickel nitrate, lithium nitrate, sodium nitrate, potassium nitrate, iron (III) nitrate, sodium nitrite, potassium nitrite, aluminum sulfate, nickel sulfate, lithium sulfate, sodium sulfate, iron (III) sulfate, ammonium sulfate, aluminum chloride, iron (III) chloride, ammonium chloride, sodium molybdate, ammonium molybdate, nickel sulfamate and ammonium sulfamate.
The present invention further provides the polishing method by means of a polishing composition, wherein the component (c) is at least one member selected from the group consisting of glycine, glyceric acid, mandelic acid, malonic acid, ascorbic acid, glutamic acid, glyoxylic acid, malic acid, glycolic acid, lactic acid, gluconic acid, succinic acid, tartaric acid, maleic acid and citric acid.